Influenza A virus is a paradigm for understanding viral entry of host cells via receptor-mediated endocytosis. We seek to deepen our understanding of viral membrane fusion in order to identify possible strategies for interfering with the process and arresting the infection cycle. The hemagglutinin (HA) protein mediates fusion of the virus and host ceil membranes. HA shares core elements with other type-1 fusion proteins such as those found in Ebola virus and HIV. While certain facets of HA-mediated fusion have been characterized, significant gaps remain in the characterization of the molecular conformations that actually drive membrane fusion, in our understanding of how HA changes conformation, and in our understanding of the interplay between fusion protein and membrane deformation. We propose to use,solution X-ray scattering with ab initio shape reconstruction to determine the structure and dynamics of the hemagglutinin fusion protein. By docking known high resolution structures of components into the solution scattering-derived shape envelopes, pseudoatomic models will be composed. This type of integrative approach is necessary for characterization of complex macromolecular machines. The experiments will provide unique insight into the conformational trajectory traversed by HA as it converts from metastable to fusion-active form. In addition, cryo-eiectron tomography will be used to elucidate the lipidic and proteinaceous architecture of the HAmediated fusion pore and to determine the higher-order organization of HA arrayed on membranes. Lastly, single-particle fluorescence microscopy will be used to reveal the dynamic fusion-activation of HA when it is arrayed on the viral membrane. This approach will identify whether the hundreds of HA spikes on the virus surface become fusion-active in a cooperative fashion.